When spacecraft first transmitted the images of the moon’s far side to Earth, we saw the lunar farside lacks the large dark areas called maria, or seas. Why?

Composite image of the lunar nearside taken by the Lunar Reconnaissance Orbiter in June 2009. Note the presence of dark areas – called maria by astronomers – on this side of the moon. Image via NASA

Composite image of the lunar farside – the side that always faces away from Earth – taken by the Lunar Reconnaissance Orbiter in June 2009. Note the absence of large dark areas. Image via NASA

The dark maria or seas – large flat areas of basalt on the moon’s near side – are sometimes referred to as the man in the moon. No such features exist on far side of the moon. Why are there dark maria on the moon’s near side, but not far side? Penn State astrophysicists think they have the answer. They believe that the absence of maria, which is due to a difference in crustal thickness between the near side of the moon and the far side, is a consequence of how the moon originally formed. The researchers reported their results in the June 9 Astrophysical Journal Letters.

Jason Wright, assistant professor of astrophysics at Penn State, said:

I remember the first time I saw a globe of the moon as a boy, being struck by how different the farside looks. It was all mountains and craters. Where were the maria? It turns out it’s been a mystery since the 1950s.

This mystery – called the Lunar Farside Highlands Problem by astronomers – dates back to 1959, when the Soviet spacecraft Luna 3 transmitted the first images of the dark side of the moon back to Earth. Researchers immediately noticed that fewer maria on the portion of the moon that always faces away from Earth.

The Penn State astronomers looked back to the formation of the moon for their ideas on why one side of the moon has maria, and the other doesn’t. The general consensus on the moon’s origin is that it probably formed shortly after the Earth and was the result of a Mars-sized object hitting Earth with a glancing, but devastating impact. This Giant Impact Hypothesis suggests that the outer layers of the Earth and the object were flung into space and eventually formed the moon.

Shortly after the giant impact, the Earth and the moon were very hot, said researchers. The Earth and the impact object did not just melt; parts of them vaporized, creating a disk of rock, magma and vapor around the Earth.

The geometry was similar to the rocky exoplanets recently discovered very close to their stars, said Wright. The moon was 10 to 20 times closer to Earth than it is now, and the researchers found that it quickly assumed a tidally locked position with the rotation time of the moon equal to the orbital period of the moon around the Earth. The same real estate on the moon has probably always faced the Earth ever since. Tidal locking is a product of the gravity of both objects.

The moon, being much smaller than Earth cooled more quickly. Because the Earth and the moon were tidally locked from the beginning, the still hot Earth – more than 2500 degrees Celsius – radiated towards the near side of the moon. The far side, away from the boiling Earth, slowly cooled, while the Earth-facing side was kept molten creating a temperature gradient between the two halves.

This gradient was important for crustal formation on the moon. The moon’s crust has high concentrations of aluminum and calcium, elements that are very hard to vaporize.

Aluminum and calcium would have preferentially condensed in the atmosphere of the cold side of the moon because the nearside was still too hot. Thousands to millions of years later, these elements combined with silicates in the moon’s mantle to form plagioclase feldspars, which eventually moved to the surface and formed the moon’s crust. The farside crust had more of these minerals and is thicker.

The moon has now completely cooled and is not molten below the surface. Earlier in its history, large meteoroids struck the nearside of the moon and punched through the crust, releasing the vast lakes of basaltic lava that formed the nearside maria that make up the characteristic man in the moon features.

Meanwhile, when meteoroids struck the farside of the moon, in most cases the crust was too thick and no magmatic basalt welled up, creating the dark side of the moon with valleys, craters and highlands, but almost no maria.

Via Penn State University

Mercury, the solar system’s innermost planet, orbits the sun inside of Earth’s orbit. Therefore, Mercury always stays close to the sun in Earth’s sky and is often lost in the sun’s glare. But Mercury reaches its greatest elongation – greatest angular distance – east of the sun on May 25, so this world can now be spotted low in the west-northeast as dusk ebbs into darkness. As always, binoculars help out with any Mercury quest.
The planet Jupiter is the first “star” to pop out after sunset. If you’re familiar with the star Regulus, you can draw an imaginary line from Regulus and past Jupiter to locate Mercury near the sunset point on the horizon. (See sky chart below.) Given a clear sky and unobstructed horizon, Mercury could be visible to the unaided eye about 60 to 90 minutes after sunset. If not, try binoculars.

Although Mercury shines more brightly than Regulus does, you might see Regulus first because it’s not as obscured by the glow of evening twilight. What is the ecliptic?

…and don’t forget the morning sky, which features the dazzling planet Venus and a thin waning crescent moon on Monday, May 26.

Setting times of the sun and Mercury in your sky

At an elongation of 23^o Mercury lies far enough east of setting sun to stay out until the end of astronomical twilight (at mid-northern latitudes). By definition, astronomical twilight ends in the evening sky when the sun is 18^o below the horizon. For reference, the sun’s diameter equals one-half degree, and your fist at an arm length approximates 10^o.

Because Mercury is setting a maximum amount of time after sunset right now, this is your chance to catch Mercury low in the west at late dusk or nightfall. But don’t tarry when seeking this elusive yet surprisingly bright world, for Mercury – even now – follows the sun beneath the horizon around nightfall. At mid-northern latitudes, astronomical twilight ends nearly two hours after sunset, at about the same time that Mercury sets beneath the horizon.

End of nautical twilight and Mercury’s setting time in your sky

We should mention that the Northern Hemisphere enjoys the better view of this particular evening apparition of Mercury. That’s because the ecliptic – the pathway of the planets – hits the horizon at a steeper angle as the sun sets in the Northern Hemisphere sky.

What is the ecliptic?

Mercury stands higher over the horizon at sunset in Northern Hemisphere than at comparable latitudes in the Southern Hemisphere. For instance, at 40^o north latitude – the latitude of Philadelphia, Pennsylvania – Mercury’s altitude at sunset is about 19^o. In contrast, at 40^o south latitude – the latitude of Wanganui, New Zealand – Mercury’s altitude is less than 11^o at sunset.

No wonder Mercury sets more than 100 minutes after sunset at mid-northern latitudes but less than 80 minutes after sunset at mid-southern latitudes. The farther north you live, the later that Mercury sets after sunset; and the farther south you live, the sooner.

Although this evening apparition of Mercury favors the Northern Hemisphere, everyone worldwide has a reasonably good chance of catching Mercury after sunset right now. Look for Mercury above the sunset point on the horizon some 60 to 75 minutes after sunset.

Mercury might be visible to the unaided eye for another week or so, but binoculars always help out with your search for Mercury, the solar system’s innermost planet.

Bruce McClure  EarthSky News

“NASA’s LADEE spacecraft captured images of the eerie zodiacal light seen from the moon, shortly before it made a planned crash landing on April 17.” As reported by EarthSky.

Check out this link to see the time lapse photo

Zodiacal light

Astronomers say that a very small asteroid entered Earth’s atmosphere on January 2, 2014, after being discovered only one day before. They’ve labeled the object 2014 AA, because it’s the first asteroid to have been discovered this year. The Minor Planet Center announced that the asteroid struck Earth’s atmosphere at around 05:00 UTC on January 2. That’s around midnight, early morning on January 2 on the U.S. East. The asteroid – which is believed to have been from about 1 to 5 meters across – is thought to have burned up over the Atlantic Ocean, probably off the coast of western Africa.

This is only the second time ever that astronomers were able to spot an asteroid before it struck Earth’s atmosphere. The first time was in 2008, when the object known as 2008 TC3 burned up over Sudan in Africa. Like 2014 AA, that object was discovered just a day before it entered Earth’s atmosphere.

Astronomers with the Mount Lemmon Survey used a 60-inch (150 cm) telescope in Arizona to spot 2014 AA on January 1, 2014 at 6:20 UTC (1:20 a.m. EST). Other astronomers quickly confirmed its presence. A calculation of the asteroid’s orbit showed it was very close to Earth, and getting closer.

According to astronomer Pasquale Tricarico, the asteroid approached the Earth from its night side. However, he said, there are large uncertainties over the time and location of the atmospheric entry of this asteroid, because only 7 observations were made while the asteroid was still visible.

Calculations by astronomer Bill Gray show that it could have entered the atmosphere between Africa and Central America.

No harm done. But the event underscores the need to watch out for near-Earth asteroids.

Bottom line: A small asteroid – probably 1-5 meters across – probably burned up over the Atlantic Ocean around midnight on the morning of January 2, 2014. Astronomers have labeled it 2014 AA.

By Deborah Byrd in EarthSky
BLOGS | EARTH on Jan 02, 2014